ENSAYO DE VARIEDADES DE EAGLE SEMENTES

In the preface to his 1981 text on steel metallurgy, Leslie [25] states that ”...as the less abundant metals become more costly...it will be essential to achieve optimum properties with minimum use of alloying elements.” While this statement may be nearly four decades old, the sentiment is as true as ever. AF9628 steel was developed in 2016 out of a desire to produce a less expensive, easier to manufacture steel for manufacturing bomb cases [11]. Dr. Rachel Abrahams of the Air Force Research Laboratory (AFRL)’s Munitions Directorate spent five months developing trying to improve the cost/performance ratio of Eglin Steel, an ultra-high strength, high tough- ness munitions steel developed with low cost in mind [7]. The goal was to improve ease of manufacturing and reduce total cost by 10%; instead, Dr. Abrahams and her team were able to create a new alloy that approached the high performance of Eglin Steel while realizing a cost savings of 50%. While this steel was developed with munitions in mind, the potential applications are boundless. It is even predicted that the high performance and low cost of this steel could inject new life into the domestic steel industry [40].

2.1.3.1 Alloy Composition

The chemical composition of AF9628 steel is free of both tungsten and cobalt, and has a relatively low percentage of nickel. Tungsten was removed from the alloy due to its high manufacturing cost; tungsten has a high melting point and high density, and creates stable tungsten carbides with even higher melting temperatures, making it difficult to melt in open furnaces [11]. A table comparing selected alloying elements of weapons steels is listed in table 2.1 below. The weapons steels prior to AF9628

were too cost prohibitive to manufacture on the scale needed for hardened target penetrating weapons, and additionally were difficult to process in thick sections [11]. AF9628 is classified as a mid-carbon steel, identified earlier (figure 2.13) as gener- ally having a high toughness. The alloy contains chromium for hardenability, strength, and temper resistance [11]. Temper resistance is the ability of a material to resist reduction in yield strength and formation of carbides while relieving internal stresses due to the stressed lattice [25]. Molybdenum was added to increase solid solution strengthening, prevent embrittlement, and improve fracture toughness. Vanadium is present to increase strength and hardenability, and control grain growth at high temperatures [11]. Vanadium will contribute to a mechanism called Zener pinning that depends on finely divided vanadium carbides to inhibit austenite grain growth [25]. Manganese provides strength [11] and deoxidation, and nickel provides low- temperature toughness by replacing iron atoms in the crystal lattice [26]. Silicon aids in the hardenability and temper resistance of the alloy by reducing the coarsening of -carbide to cementite. By preserving the smaller, semi-coherent -carbides, silicon also enhances toughness. Copper is considered an undesirable inclusion in this al- loy; if it is present above 0.20% by weight it will precipitate out and reduce fracture toughness [11].

Table 2.1. Comparison of alloy composition of several weapons steels

Alloy Name C Ni Cr Mo Co Mn V W Si AF1410 [41] 0.15 10 2 1 14 - - - - Aermet-100 [41] 0.24 11.5 3.1 1.2 13.5 - - - - HY-180 [42] 0.13 10 2 1 8 0.10 - - 0.05 HP 9-4-30 [43] 0.30 7.5 1.0 1.0 4.5 0.30 0.10 - 0.10 Eglin Steel [7] 0.28 1.03 2.75 0.36 - 0.74 0.06 1.17 1.00 AF9628 [11] 0.28 0.95 2.52 0.91 - 0.62 0.064 - 0.96

2.1.3.2 Strengthening Mechanisms

AF9628 is believed to be strengthened by special meta-stable nano-carbide fila- mentous rods that are precipitated within a primarily martensitic matrix. Specifi- cally, the combination of alloying elements and heat treatment would likely lead to the mixed precipitation of metastable -carbide and cementite. However, the actual pres- ence of these -carbides has been difficult to confirm due to their extremely small size. The potential -carbides are nano-sized, iron-rich, metastable, and semi-coherent, which leads to increased strength and reduced loss of toughness [11]. AF9628 is typically void of tool-steel alloy carbides such as M23C6, M2C, or M6C. The matrix

is intentionally kept to 90% martensite or greater, in contrast to most other high- strength Ni-Cr-Mo steels [11]. Values for strength, hardness, ductility, and toughness for AF9628 and five other munitions steels are provided for comparison in table 2.2. The potential -carbides, shown in Figure 2.15 are 100nm to 150nm in length and 10nm in width. Their shapes are similar to feathery rods and fit within the matrix (semi-coherent), causing localized strain features. The configuration of -carbide is believed to be Fe2/4C, hexagonal close packed. -carbide is thermally unstable, so

tempering is performed at temperatures less than 260◦C. The semi-coherent dis-

tribution, rather than at the grain boundaries, promotes the favorable combination Table 2.2. Comparison of material properties of several weapons steels

Alloy Name Ultimate Tensile Strength Yield Strength Elongation to Failure Charpy Impact Toughness Hardness

ksi ksi % ft-lb at -40C Rockwell C

AF1410 [41] 235 215 12 Aermet-100 [41] 280 235 8 HY-180 190 175 12 HP9-4-30 232 194 15 20 51 Eglin Steel [7] 260 230 17.5 20 46 AF9628 [11] 230 180 11 24 45

of strength and dynamic toughness [11]. AFRL/RX has attempted to confirm the presence of the -carbides with higher-resolution Transmission Electron Microscopy (TEM), but has not yet achieved success.

Figure 2.15. SEM image taken at 60,000x magnification showing what may be nanoscale -carbide within a primarily martensitic matrix grain structure [11]

2.1.3.3 Processing Characteristics

AF9628 may be significantly easier to process than other weapons steels, most notably in that it can be produced in an open ladle process. It is also easily welded and machined, especially in the annealed condition, as it does not contain a large number of carbides in that state [11]. The alloy can be cast or wrought, and shape casts can be further treated using a Hot Isostatic Press (HIP) process. The declared material processing characteristics of the six munitions steels of note are tabulated below (table 2.3 for comparison.

The suggested heat treating process begins with an austenitizing step, heating to above 954◦C for 30 minutes per inch of thickness. The heating rate between

Table 2.3. Comparison of manufacturing qualities of several weapons steels

Alloy Name Melt Process Weldability Machinability Hardening Process AF1410 [41] Double Vacuum

Melted

Good, no pre- heat

Multiple heat and air quench steps, refrig- eration, and aging Aermet-100 [41, 44] Double Vacuum Melted Good, no pre- heat More difficult than 4340

Crucial air or oil quench, refriger- ation, and aging HY-180 Double Vacuum

Melted Good HP 9-4-30 [43] Consumable Electrode Vacuum Arc Re-melting Good, helium shielded tig

Similar to 4340 Oil quench

Eglin Steel [7] Electric Arc, La- dle Refined, Vac- uum Treated

Excellent Similar to 4340 water, oil, or gas quench, low heat temper

AF9628 [45] Ladle Excellent Similar to 4340 water quench, low heat temper

carbides. The steel should then be rapidly quenched to below 66◦C; the value of

the Grossman H-value for quench intensity should be above 0.25. Sections over one inch thick should be quenched in water, as an oil quench will be too slow to prevent the formation of undesirable carbides. A tempering step performed between 177 and 260◦C for a minimum of three hours (or one hour per inch of thickness) will improve

toughness. The AF9628 patent recommends a sub-critical step to decrease the size of prior austenite grain boundaries, however, in work presented by Sinha at Materials Science & Technology (MS&T) 2018, the sub-critical anneal produced no measureble change in PAGB size.